This invention relates to a structure for mounting a thermistor with positive resistance-to-temperature characteristic. More particularly, this invention relates to thermistors with improved voltage resistance.
Thermistors with positive resistance-to-temperature characteristics (hereinafter referred to simply as the positive characteristic) are used, for example, in overcurrent protection circuits in the telecommunication field. FIGS. 17, 18 and 19 show a prior art structure for mounting a positive characteristic thermistor 1 for such a use, FIG. 17 being a plan view of a portion of a hybrid IC having the thermistor 1 mounted thereto, FIG. 18 being a sectional view taken along line XVIIIxe2x80x94XVIII in FIG. 17 and FIG. 19 being a sectional view taken along line XIXxe2x80x94XIX in FIG. 17.
As shown in FIGS. 17-19, the positive characteristic thermistor 1 has electrodes 2 and 3 formed respectively on its mutually opposite main surfaces. This thermistor 1 is mounted to a substrate 4 made, for example, of a ceramic. The substrate 4 has conductive members which are each electrically connected to a corresponding one of electrodes 2 and 3. According to this prior art example, the conductive members include conductive lands 5 and 6 formed on the substrate 4, a connecting member 8 to be connected to the conductive land 6 through solder 7, a solder part 9 provided so as to electrically connect the conductive land 5 and the electrode 2, and another solder part 10 for electrically connecting the connecting member 8 and the electrode 3. Lead terminals 11 and 12 are also attached to the substrate 4 for electrically connecting to the conductive lands 5 and 6, respectively.
The resistance against voltage by a thermistor 1 thus structured sometimes deteriorates, depending upon how it is mounted, compared to before it is mounted. This is because the escape of heat from the ceramic substrate 4 is relatively high such that, when the electric energy applied to the thermistor 1 is converted into heat, a large portion of this heat energy escapes through the solder parts 7, 9 and 10, as well as the connecting member 8, thereby increasing the heat flow from the thermistor 1 and destroying its balance.
It is therefore an object of this invention to provide an improved mounting structure for a thermistor with positive characteristic capable of preventing the deterioration of its voltage resistance.
This invention relates to a structure for mounting a thermistor with positive characteristic having electrodes on mutually opposite main surfaces to a substrate with electrically conductive members to which the electrodes are to be electrically connected. In order to attain the above and other objects, the structure comprises means for restraining heat conduction from the thermistor to the electrical conductor, characterized by a small cross-sectional area for electrical conduction.
In examples wherein the electrically conducting members comprise electrically conductive lands soldered to the electrodes for electrically connecting, the conduction-restraining means comprises a spacer which is made of a material with lower thermal conductivity than the substrate and disposed between a conductive land and an electrode and the portion of the conductive member with reduced cross-sectional area penetrates this spacer. According to another embodiment of the invention, a portion of the conductive land is covered with a non-metallic material such that the area on which solder is applied can be limited. According to still another embodiment of the invention, the portion of the conductive member with reduced cross-sectional area is realized by means of a chip-like connecting member soldered to the electrode.
The electrically conductive members of this invention may be disposed opposite to the electrodes and provided with protrusions that contact the electrodes such that these protrusions can function as narrowed contacting parts with reduced cross-sectional area.
If the thermistor is disposed such that one of its electrodes faces the substrate, the part of one of the electrically conductive members contacting the other of the electrodes may be formed with a cross-sectional surface transverse to the longitudinal direction of the conductive member so as to reduce the area of contact to the electrode. Alternatively, the conductive member may be bent such that the bent edge serves to contact the electrode.
According to this invention, a means for restraining thermal conduction, including cross-sectionally narrowed portion, is provided near or at the position of contact between an electrode on the thermistor and an electrically conductive member on the substrate such that emission of heat from the thermistor can be controlled. Deterioration of voltage resistance can thus be controlled when a thermistor is mounted to a substrate.